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Hauling hay and a gas engine question

S

SVcummins

Well-known Member
Why does the old 72 gmc backfire when you shut it off ? Been
hauling hay today and it’s backfired twice and one time it
didn’t
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High idle pumps fuel and air onto exhaust when you turn it off. Also helps to let it idle for a minute or more before shutting it off.
 
Some of this vintage had a small solenoid that controlled low idle, and allowed the throttle plates to close completely when the key was shut off, and they would still occasionally diesel, or back fire.
 
To make it stop, set the idle as low as you can and still have it running OK. As the engine slows to a stop from its current idle speed, the spark quits, but the fuel and air are now pumped into the exhaust. ANy glowing bit of carbon in the manifold, the back of an exhaust valve, or any small piece of hot metal will set it off with a bang. To fix it without any further effot, Put it in high gear and let the clutch out as you shut it off. This quickly stops engine rotation and therefore no fuel/air to go BooM. Jim
 
Reminds me of a old ford F 700 I had several years back...I was driving in the rough section of town and went to stop at a traffic light. On the corner was a car wash with a good crowd hanging out, truck backfired and crowd hit the ground.
 
My SMTA does this. It ain't broke, so I don't see the need to fix anything. I reduce speed to and idle and let it sit for most of a minute while I take a breather and look at all my good work. Then I shut it down. Rarely backfires if I give it a minute to idle before shutting her off. Worth a try.
 
We'd wind up our old '46 Ford pickup, turn off the igntion, then turn it back on just before it stopped. Back fire would blow the muffler off about half the time. Just what my brother and I wanted.
 
Have idle set below 700 rpm. Save the muffler and stall it in high at the same time as turning key off. GM has solenoids on the carb. You set the throttle plates closed with the manual screw and set the idle with the solenoid. When key is turned off solenoid is deactivated and throttle plates are completely closed.
 
I did that one time and one time I Left the key with the truck in gear and coated a ways when I turned the key back it blew the side out of the block and what looked parts of the cam shaft
 
Bingo, Roger.

Idle speed is too high.

Can also cause run-on caused by dieseling when plugs are hot.

Dean
 
Leaky exhaust has a greater effect on causing deceleration back fires more then shutdown back fires but can perpetuate either. As stated the air is already mixed in with the and fuel by the carb, then just passes through the cylinders when the spark is cut off. I think this model is still from the era prior to the introduction of anti-diesel solenoids.
 
A couple things can contribute to it.

Exhaust leaks, cracked manifold, remains of AIR emissions, or aka a "smog pump", or a natural draft air supply to the exhaust manifolds will supply needed air to make one backfire on shut off and deceleration.

Retarded ignition timing, bad vacuum advance, frozen centrifugal advance, will cause higher exhaust temps, and require the throttle idle stop to be too far open to keep it from dying.

Vacuum leaks, Check the PCV hose, see if the underside is sucked in at the barb fitting and leaking.

Try to set the idle as low as practical, and kill it with the clutch as soon as you cut the key. That will keep the engine from pumping raw gas into the exhaust as the engine is coasting down.
 
(quoted from post at 00:11:42 01/05/21) A couple things can contribute to it.

Exhaust leaks, cracked manifold, remains of AIR emissions, or aka a "smog pump", or a natural draft air supply to the exhaust manifolds will supply needed air to make one backfire on shut off and deceleration.

Retarded ignition timing, bad vacuum advance, frozen centrifugal advance, will cause higher exhaust temps, and require the throttle idle stop to be too far open to keep it from dying.

Vacuum leaks, Check the PCV hose, see if the underside is sucked in at the barb fitting and leaking.

Try to set the idle as low as practical, and kill it with the clutch as soon as you cut the key. That will keep the engine from pumping raw gas into the exhaust as the engine is coasting down.
Click to expand...

Steve@Advance has it well covered, all I can add is pull a few spark plugs to see if they look like they are running lean/hot.

Dirty carbs and vacuum leaks will contribute to a lean mixture.

Glowing clinkers of carbon from higher than normal exhaust temperatures become the ignition source for the bang.
 
To haul another bale get a set of old loader arms,turn them upside down on the rear of the truck bed, let them hang off the back some, setting up in the air,chain the arms on the truck down with a chain and load binder.The 3rd bale will wedge them all in place.
 
Hauled some more hay today if I let the truck idle while take the straps off then it doesn’t backfire when I shut it off
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I have also been thinking or extending the bed I could go 3 feet and still miss the ground with the bed dumped all the way and that would easily fit another tier of 3 bales
 
Thanks . Sure wish or agriculture technology school would
had the foresight to realize there are engines on a farm
besides diesel
 
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